1. Field of the Invention
The present invention relates to an information processing terminal, IC card, portable communication device, wireless communication method, and program.
2. Description of the Related Art
In recent years, contactless information processing terminals that can communicate with a read/write device (referred to as “reader/writer”, hereinafter) through wireless communication, such as contactless integrated circuit (IC) cards and radio frequency identification (RFID) tags, have been widely distributed.
Contactless IC cards and RFID tags do not have a power supply in these media themselves but employ an induced voltage as the driving power supply thereof. The induced voltage refers to a voltage that is generated depending on magnetic flux when a magnetic field arising due to flowing of a current through a transmission coil as a transmission/reception antenna included in a reader/writer passes through a reception coil as a transmission/reception antenna included in the contactless IC cards and RFID tags. That is, the contactless IC cards and RFID tags receive through the reception coil the energy of the magnetic field transmitted from the transmission coil of the reader/writer, to thereby acquire the power necessary for driving thereof.
Furthermore, the contactless IC cards and RFID tags enable/disable a load included therein to thereby vary the impedance seen from the reader/writer. This impedance variation for the reader/writer through the enabling/disabling of the load is referred to as load modulation.
The load modulation in the contactless IC card or RFID tag varies the current that flows through the transmission coil in the reader/writer. The reader/writer detects the change of the current flowing through its transmission coil, varied due to the load modulation of the contactless IC card or RFID tag. The reader/writer regards this current change as response from the contactless IC card or RFID tag, and operates depending on the response.
In general, as the distance between the reader/writer and the contactless IC card or RFID tag becomes larger, the intensity of the magnetic field received by the reception coil of the contactless IC card or RFID tag from the reader/writer becomes smaller, and the induced voltage also becomes lower.
Furthermore, a magnetic field with a specific frequency (referred to as “carrier”, hereinafter), such as a frequency of 13.56 MHz, is utilized for the communication between the contactless IC card or RFID tag and the reader/writer. The contactless IC cards and RFID tags include a resonant circuit designed to resonate at the specific frequency. However, if an impeder such as another contactless IC card exists between the reader/writer and the contactless IC card or RFID tag, the resonant frequency is changed, which lowers the induced voltage.
For example, Japanese Patent Laid-open No. 2001-222696 and No. Hei 10-233717 disclose a technique for stabilizing communication between an information processing terminal such as a contactless IC card or RFID tag and a reader/writer even when the distance between the information processing terminal and the reader/writer is changed or the resonant frequency of the information processing terminal is changed.
However, in some cases, the load modulation would be possibly executed when the energy of a magnetic field acquired by the information processing terminal from the reader/writer is low, i.e., when the magnitude of power acquired by the information processing terminal from the magnetic field is small, such as when the Induced voltage is originally low or when resonance is not caused at the specific frequency used as the frequency of the carrier and thus the induced voltage is lowered. In these cases, the information processing terminal would possibly become inoperative due to failure in acquisition of the power necessary for driving of the information processing terminal because of power consumption of a load such as a resistor relating to the load modulation. This would possibly lead not only to failure in the communication between the reader/writer and the information processing terminal but also to other various problems, such as corruption of data relating to the communication and recognition of unexpected data by the reader/writer.
Problems in an existing information processing terminal will be described below based on FIG. 6. FIG. 6 is an explanatory diagram showing a circuit relating to load modulation in the existing information processing terminal.
Referring to FIG. 6, the circuit relating to load modulation in the existing information processing terminal is formed of a resistor Rf having a specific resistance R and an N-channel metal oxide semiconductor (MOS) (referred to as “NMOS transistor”, hereinafter) Tr2. The NMOS transistor Tr2 serves as a switch that is turned ON/OFF based on a response signal S5 that is a binary signal indicating a high or low level. An induced voltage V1 is applied to one end of the circuit. When the NMOS transistor Tr2 is ON, i.e., when the response signal S5 is at the high level, the other end of the circuit is connected to the ground GND, so that a current I4 flows through the circuit based on the induced voltage V1 and the resistance R of the resistor Rf. In contrast, when the NMOS transistor Tr2 is OFF, i.e., when the response signal S5 is at the low level, the circuit is open, and hence the current I4 does not flow through the circuit. The current I4 that flows through the circuit is expressed as I4=V1/R, and the power P3 consumed by the circuit is expressed as P3=R(I4)2=(V1)2/R.
As the resistance R of the resistor Rf becomes lower, the current I4 that flows through the resistor Rf when the response signal S5 is at the high level becomes larger, and the power consumption P3 also becomes higher. When the current I4 is large, a variation in the impedance of the existing information processing terminal seen from a reader/writer is also large, which has an advantageous effect on signal transmission from the existing information processing terminal to the reader/writer. However, the power consumption P3 is also high at this time. Therefore, if the induced voltage V1 is low, there is a possibility that the existing information processing terminal cannot be driven normally due to failure in securing of the power necessary for driving of the existing information processing terminal. When the information processing terminal is an IC card in particular, the terminal does not have a power supply in the terminal itself in many cases. Therefore, the failure in securing of the power necessary for the driving would lead to a serious problem.
In contrast, if the resistance R of the resistor Rf is increased, the current I4 that flows through the resistor Rf and the power consumption P3 can be decreased. In this case, the possibility of the occurrence of the failure in securing of the power necessary for driving of the existing information processing terminal is low. However, the variation in the impedance of the existing information processing terminal seen from the reader/writer is small irrespective of the magnitude of the induced voltage V1, which has a disadvantageous effect on signal transmission from the existing information processing terminal to the reader/writer.